Characterization of POU2F3-expressing large cell neuroendocrine carcinoma of the lung: A comprehensive analysis of morphology, immunohistochemistry, and genomic alterations

• Distinct entity – By using histology, immunohistochemistry, whole-genome sequencing, and a reanalysis of a 75-case public dataset, we herein demonstrated that LCNEC-P forms a cohesive biological group with SCLC-P that is clearly separate from other LCNEC/SCLC subtypes. • Diagnostic impact – We confirmed POU2F3 immunostaining as a practical marker and delineated pitfalls, such as basaloid squamous cell carcinoma with POU2F3 positivity, offering a concrete diagnostic algorithm. • Therapeutic insight – We observed a relatively high frequency of FGFR1 amplification (60%) in LCNEC-P, indicating a possible therapeutic relevance in this subset. • Clinical relevance – NEC-P has significantly poorer 5-year overall survival and is closely associated with RB1 inactivation, indicating the need for tailored management. Large cell neuroendocrine carcinoma (LCNEC) is a neuroendocrine carcinoma (NEC) of the lung that is characterized by its heterogeneous morphology, diverse immunophenotypes, and complex genomic profiles. Among LCNECs, a subset expressing the transcription factor POU2F3 (LCNEC-P) has been suggested to share similarities with small cell lung carcinoma (SCLC)-P, a subtype of SCLC defined by POU2F3 expression. However, the specific characteristics of LCNEC-P have not been fully elucidated. Therefore, the aim of the present study is to clarify the clinicopathological, immunohistochemical, and genetic characteristics of LCNEC-P. Fifty-six LCNEC cases were analyzed, including 12 LCNEC-P and 44 LCNEC-non-P cases. Morphologically, LCNEC-P exhibited significantly lower cytomorphology scores, indicating a resemblance to SCLC. Immunohistochemically, LCNEC-P showed the lower expression of neuroendocrine markers (SYP, CHGA, and INSM1), but the higher expression of C-MYC than LCNEC-non-P. A strong mutually exclusive expression pattern was observed between POU2F3 and ASCL1/NEUROD1. Whole-genome sequencing of 20 cases revealed that LCNEC-P harbored RB1 mutations in 100% of cases, which was significantly higher than in LCNEC-non-P (40%). FGFR1 amplification was observed in 60% of LCNEC-P cases, representing a higher prevalence than previously reported for LCNEC. In addition, LCNEC-P showed a distinct copy number alteration profile, including frequent 20q13 amplification, compared with LCNEC-non-P. These results demonstrate that LCNEC-P represents a distinct subgroup of LCNEC that is characterized by a specific morphological, immunohistochemical, and genetic profile, closely resembling SCLC-P. This study provides insights into the biology of LCNEC-P and supports its classification as a unique entity within LCNEC.